Induction-based food holding/warming system and method

ABSTRACT

A system and method for holding and warming food held in pans or trays having a low relative magnetic permeability, such as 300-series stainless steel, using induction heating.

BACKGROUND OF THE INVENTION

The present invention generally relates to typical food service,food-holding/warming systems employing a hot water steam bath with foodtray/dish inserts. This system replaces the water/steam approach with aninduction heating system.

Typical food service, food-holding/warming systems use food tray/dishinserts heated from below by a hot water steam bath, termed a “steamtable.” Steam tables have proven effective, but have severaldisadvantages. Given the thermal inertia of the water, the steam isrelatively slow to heat the food, and the temperature can be difficultto predict and control. Replenishment of the water, and particularlyavailability of gas if the steam tables are gas-driven, can also be anissue given the locale (e.g., an airport, remote location, etc.). Otherdisadvantages of steam table systems are the requirement for a watersupply and associated drainage. Additionally, the induction system issubstantially more energy efficient than the hot water/steam approach.

Food holding/warming systems typically use a variety of stainless steelpans, trays, bowls, etc. Existing steel pans are typically made ofAustenitic 300-series stainless steel (particularly 303 stainless)because of its corrosion resistance, a NSF requirement. These are,commonly known in the cooking industry, as well as from aphysics/materials point of view, as being non-magnetic. Inductionheating/cooking inherently assumes and relies upon the use of magneticmaterial in pots, pans, etc. For example, if stainless steel is used,then induction heating/cooking would presume the use of cookware thoughtof as “magnetic,” such as 400-series stainless steel.

For these reasons, food service pans, typically made of 300-seriesstainless steel (specifically, 303), has not been heated using inductionheating methods. This appears to have led the induction-cooking productindustry to assume that an inherent incompatibility exists betweenservice pans and the use of induction heating.

Additionally, in food holding/warming applications, it is often desiredto use equipment that is as quiet as possible, given the presence ofcustomers in this environment. Virtually all induction cooking equipmentemploys fans for cooling of the electronics, given the heat generated bythe inverters and induction coils. The elimination of the need for fansis another advantage of the present invention.

Accordingly, it would be advantageous to provide apparatus, systems anda method for holding and warming food which is quiet and which providesefficient, fast and predictable heating control.

DEFINITION OF CLAIM TERMS

The following terms are used in the claims of the patent as filed andare intended to have their broadest meaning consistent with therequirements of law. Where alternative meanings are possible, thebroadest meaning is intended. All words used in the claims are intendedto be used in the normal, customary usage of grammar and the Englishlanguage.

“Magnetic permeability” means the measure of the ability of a materialto support the formation of a magnetic field within itself, i.e., thedegree of magnetization that a material obtains in response to anapplied magnetic field. Permeability is the inductance per unit length.In SI units, permeability is measured in henries per metre (H m⁻¹), ornewton per ampere squared (N A⁻²).

“Relative magnetic permeability” means the ratio of the magneticpermeability of a material to the permeability of a vacuum, which is1.0.

SUMMARY OF THE INVENTION

The objects mentioned above, as well as other objects, are solved by thepresent invention, which overcomes disadvantages of prior foodwarming/holding systems and method, while providing new advantages notpreviously obtainable with such systems and methods.

One preferred embodiment includes a system and a method for holding andwarming food, in which a pan or tray for holding and warming the food ismade of a material with a low relative magnetic permeability, such asless than about 20, such as cold-worked, 303 stainless steel, and inwhich the pan or tray is heated using an induction heating unit. In onepreferred step of induction heating, an induction inverter is used whichoperates in a preferred frequency range of about 40-60 kHz.

Preferably, the induction heating circuit includes an induction coilhaving a sufficiently large number of coil turns so as to increase themagnetic field strength to a level sufficient to induce a desiredheating temperature into the pan or tray. Most preferably, the inductioncoil may have in the range of about 40-80 turns, and a wire gauge in therange of about 14-18 gauge.

Most preferably, the step of induction heating occurs at a sufficientlylow coil current to minimize internal heat generation in the inductionunit, enabling induction heating without the need for a cooling fan. Forexample, the current in the induction coil may be less than about 15amperes.

The temperature of the pan or tray may be measured using a temperaturesensor such as a resistive thermal device (RTD). The measuredtemperature may be used in a closed loop control to limit and maintainthe temperature of the pan or tray at a desired value. Preferably, themeasured temperature may be used to generate a ramped inverter powerprofile, avoiding the generation of hot spots and facilitating lateralheat transfer in the pans or trays.

Preferably, the RTD may be mounted beneath the pan or tray in a mannerto facilitate maintaining contact with the pan or tray despitevariations in shape or size of the pan or tray.

In one preferred embodiment, the induction heating unit includes aninverter power circuit, power circuit controls, and an inverter resonantcircuit having an induction coil and a resonant capacitor. In apreferred embodiment, the induction coil has about 40 or more turns whenusing 120 volt coils, and has about 75 or more turns when using 240 voltcoils. In this embodiment, the inductance of the heating unit coil maybe about 250 μHenries or more when using 120 volt coils, and about 800Henries or more when using 240 volt coils. The inverter resonant circuitmay be about 5-11 amps for 120 volts, and about 4-8 amps for 240 volts.

In another preferred embodiment, a system for warming food in a pan ortray made of 300-series stainless steel by induction heating isprovided. This system may include an induction unit with a mastercontroller. One or more remote control units including one or more slaveunits may be provided, with each of the one or more remote control unitsbeing associated with one of the pans or trays. Serial communicationarchitecture may be provided, which may include a communication busenabling the transfer of data between the master and slave units. Themaster controller may be enabled to induct heat to a series of the pansor trays in a controlled fashion using one or more of the slave units.

In a particularly preferred embodiment of this system, the communicationarchitecture may use at least four data lines, at least one of which isbi-directional, and at least first and second control signals. Thecommunication bus may use a standard Ethernet cable to connect themaster controller to each of the one or more slave units. The firstcontrol signal may latch the address of the data being sent by themaster controller to the one or more slave units. The second controlsignal may latch the actual data being sent by the master controller tothe one or more slave units. When the first and second control signalsare in an opposing state, the master controller can use at least threeof at least the four data lines to request that one bit of data bereturned from one or more of the slave units. At least three of the datalines may be used to select a bit of date requested by the mastercontroller. The slave units may include a programmable device thatallows discrete logic to be configured internally and combined with anintegrated microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are characteristic of the invention are setforth in the appended claims. The invention itself, however, togetherwith further objects and attendant advantages thereof, will be bestunderstood by reference to the following description taken in connectionwith the accompanying drawings, in which:

FIG. 1 is a block diagram of an inverter and controls which forms apreferred embodiment of the present invention;

FIG. 2 is a timing diagram illustrating a typical serial bus transactionwhich may occur using exemplary architecture for the communicationprotocol between a master controller (main induction unit) and its slave(remote control box), forming a preferred embodiment of the presentinvention;

FIG. 3 is a schematic diagram illustrating slave hardware which may beused to utilize the bus signals, according to a preferred embodiment ofthe invention;

FIG. 4 is a flow chart illustrating an exemplary process for the mastercontroller for transmitting data on the bus, according to a preferredembodiment of the invention;

FIG. 5 is a schematic diagram illustrating one embodiment of slavehardware for using the bus signals;

FIG. 6 is a schematic representation of an embodiment illustrating aninternal configuration of a programmable device required to constructthe slave hardware, according to a preferred embodiment of theinvention;

FIG. 7 is a top/side perspective view of a rectangular induction heatingunit including exemplary RTD assemblies in a preferred embodiment of theinvention;

FIG. 8 is a top/side perspective view similar to FIG. 7 and also showinga typical food holding/warming tray/pan;

FIG. 9 is a top/side perspective view showing a portion of preferredinduction heating units useful for the present invention, including theinverters and induction coils;

FIG. 10 is a front/top perspective view illustrating a round inductionheating unit;

FIG. 11 is a front/top perspective view showing the induction heatingunit in FIG. 10 together with a typical round pan/tray forholding/warming food;

FIG. 12 is a partial front/top perspective showing a preferred inverterinside a round induction heating unit removed from its enclosure;

FIG. 13 is a partial, enlarged perspective view of an assembled RTDtemperature sensor mounted to a round induction heating unit;

FIG. 14 is an enlarged front perspective parts view of the RTDtemperature sensor assembly shown in FIG. 13;

FIG. 15 is a partial front perspective view of the RTD temperaturesensor assembly shown in FIG. 14, together with its preferred pleatedrubber seal;

FIG. 16 is a partial, enlarged perspective view showing an exemplaryparts assembly inside a preferred round induction heating unit; and

FIG. 17 is a rear perspective view of the slave hardware shown in theschematic diagram of FIG. 3.

The components in the drawings are not necessarily to scale, emphasisinstead being placed upon clearly illustrating the principles of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Set forth below is a description of what are believed to be thepreferred embodiments and/or best examples of the invention claimed.Future and present alternatives and modifications to this preferredembodiment are contemplated. Any alternatives or modifications whichmake insubstantial changes in function, in purpose, in structure, or inresult are intended to be covered by the claims of this patent.

It has been discovered that the cold working process of forming pans outof 300-series stainless material (particularly 303 stainless) includesthe side effect of making the material slightly magneticallysusceptible. This slight magnetism is not sufficient for effectiveinduction heating using conventional induction methods. However, it wasalso discovered that by modifying the induction inverter, induction maybe used to more efficiently heat pans made of 300-series stainless steel(such as, specifically, 303 stainless), particularly when only warmingor holding is required. The method generally consists of twoelements: 1) operating at a higher frequency than most induction cookinginverters employ (i.e., while typical induction cooking involvesoperating at 20 khz-30 khz, it was found that operation in the 40 khz-60khz region results in greater losses (more heating) in the pans); and 2)employing greater numbers of coil turns, yet combined with lowercurrents to generate stronger magnetic fields with the induction coilthan typically used, also generating greater losses in the pans.

In electromagnetism, permeability is the measure of the ability of amaterial to support the formation of a magnetic field within itself. Inother words, it is the degree of magnetization that a material obtainsin response to an applied magnetic field. Permeability is the inductanceper unit length. In SI units, permeability is measured in henries permetre (H m⁻¹), or newton per ampere squared (N A⁻²). Relativepermeability is the ratio of a material's permeability to thepermeability of a vacuum, which is 1.0. Materials used forinduction-compatible cooking vessels have relative permeabilities in therange of 100 to a few thousand. Non-cold-worked 303 stainless steel hasa relative permeability near 1, while cold work processing may increaseits permeability in the range of 5 to 20. For example, the permeabilityof cold worked 303 stainless steel is typically one tenth to onetwentieth that of 430 stainless steel, which often is used as aninducible material in cookware.

The method of the present invention used to eliminate noisy fanstypically employed in conventional induction heating systems is tosignificantly lower the current in the resonant tank of the inductionunit/inverter. Heating of a pot or pan is the result of current flowingin the pot/pan, which is in turn induced by the coil in the inductionunit. The higher the current, the greater the heating. From the point ofview of the induction unit and its coil, the current induced in thepot/pan is proportional to the magnetic field intensity. This, in turn,is in proportion to the product of coil current (amps) and coil turns(T) or ampere-turns. Thus, the same field strength can be created by avariety of pairings of current and coil turns.

Conventionally, induction units are designed with high coil currents,and as few turns in the coil as possible. Heat, in the electronics andcoil, is primarily the result of current, as well as switching losses.The inverter we used has virtually no switching losses, and minimizesthe resonant circuit current. Thus, to generate a sufficiently strongfield, the number of coil turns is greatly increased as compared totypical units. This also allows thinner, cheaper coil wire to be used,as well as less expensive resonant capacitors in the inverter. The netresult is that so much less heat is generated by the electronics, thatno fan is required to cool them, providing a system that runs morequietly and efficiently.

Referring to FIG. 1, a schematic diagram showing one preferred invertercircuit 10 including various controls is provided. Thus, for heating apan or tray 20, inverter circuit 10 may be used, and may generate amagnetic field (as shown by the double arrows) to heat the pan or tray20. Inverter circuit 10 may include: inverter power circuit 34(including induction coil 30, resonant capacitor 32 and power circuitcontrols 35); bridge rectifier 37; line filter 39, connected to AC powerlines 40 and 41; low voltage power supplies 42; processor/controller 44;temperature sensor/RTD 46; and user interface/display 48. Referring toFIG. 16, an exemplary induction heating unit shows various of theseelements as assembled.

In a preferred embodiment, for 120 volts, 40 turns of coil were usedwith 14-gauge wire (165 strands of 36-gauge copper Litz wire, useful forhigh frequencies), with an inductance of 256 μHenries. For 240 volts, 75turns of coil were used with 18-gauge wire (65 strands of 36-gaugecopper Litz wire), with an inductance of 820 μHenries. Typical inductioncooking appliance designs will use 120 volt coils with less than 20turns and inductances typically less than 100 μHenries, and 240 voltcoils with less than 30 turns and inductances typically less than 150μHenries. A useful range of current for the resonant tank of theinverter was found to be 5-11 amps for 120 volts, and 4-8 amps for 240volts. Typical induction cooking appliance designs will have coilcurrents in the rage of 25-50 amps or more.

Referring to FIGS. 7-12 and 16, induction heating units 10 useful inboth rectangular and round form are shown, together with correspondingrectangular and round pans/trays 20. As shown in FIGS. 9 and 12,induction heating units 10 may include induction coils 30 and inverterpower circuits 34. FIG. 16 shows round induction unit 10, including itsinverter 34, its line filter 39, bridge rectifier 37, power transistors47 for the power circuit 35, resonant capacitor 32, low voltage powersupplies 42, and control processor 44.

Referring now to FIGS. 10 and 13-15, to measure pan or traytemperatures, an RTD (Resistive Thermal Device) temperature sensor 46may be used, and may be mounted beneath pan or tray 20. A particularlypreferred RTD temperature sensor 46 may be housed in a metal can 52(FIG. 15), which, in turn, may be attached to a pleated rubber gasketseal 53 (FIGS. 13-15). Gasket 53 may be a thin pleated disk, providingboth a seal against food and other intrusions, as well as providing aflexible mounting for RTD 46, such that it can freely move up and down.RTD 46 and pleated seal 53 may additionally be mounted in an assemblywhich includes a spring 54 located underneath the RTD and above mountingplate 57 (FIG. 14, rubber seal 53 removed from mounting plate 57 in thisview), allowing the RTD to move up and down while still maintainingcontact with the bottom of pan or tray 20. This allows the RTD tomaintain pan/tray contact despite variations in the size and shape ofthe pans/trays and their positioning. Two manufacturers of RTDs that maybe used, for example, are Sensortec, Inc. and Heraeus Sensor TechnologyUSA.

The RTD may then operate in a closed loop with pan/tray 20 and theinduction unit and its processor and controls (FIG. 1), to maintaindesired pan/food temperatures and temperature profiles. By “closed loop”it is meant that the temperature measured by the RTD is used by controlprocessor 44 to determine the on/off and power output level operation ofthe inverter 34, which, in turn, determines the heating of pan/tray 20.Processor 44 may do so with an algorithm that operates inverter powercircuit 34 to achieve a particular temperature profile. For example, thealgorithm can be as simple as a comparison to determine whether thetemperature is below a threshold temperature, in which case the inverteris instructed to run, or above the threshold, in which case the inverteris turned off. More complicated algorithms may be used if, for example,anticipatory controls are used, custom-designed for particular foodcharacteristics. Particularly in heating 303 stainless steel pans withfood to a desired temperature, it is advantageous to ramp the powerslowly and accurately to avoid hot spots and allow for adequate lateralheat transfer.

It is preferred that systems of the present invention be able tomaintain accurate temperature control at low power levels. The low powerlevels are required to provide even heating of a 300-series stainlesssteel pan/tray, or pans/trays made of materials with comparable relativemagnetic permeabilities. Without accurate control, the pan/tray maydevelop severe thermal gradients that can render the temperature holdinginadequate. One preferred method disclosed here is to employ astepped-profile, using multiple thresholds and inverter power levelsbased on the measured temperature of the pan/tray.

For induction units with a remote control/user interface, a serial busmay be desired to provide a communication protocol between the mastercontroller (main induction unit) and the slave (remote control bus). Theexisting serial communication architecture is not desirable due to cost,complexity or speed requirements. Accordingly, a custom architecture wasdeveloped to provide the following (which may of course be varied,depending upon system needs):

-   -   1. Transfer 32 bits of data to be sorted into 4 blocks of 8 bits        each from the master to the slave.    -   2. Transfer 8 bits of serial data from the slave to the master        using one of the data lines for sending data to the slave.    -   3. Perform all of this in less than 10 microseconds.    -   4. Minimize the number of wires required between master and        slave—preferably 6 or fewer.

The architecture developed uses four data lines, one of which isbi-directional, and two control signals. This allows the bus to use astandard Ethernet cable to connect the devices.

The first control signal, AL, may latch the address, or segment of the32 bits of data being sent on the positive edge of the signal. Thesecond control signal, DL, may latch the actual data for that segment onthe negative edge of the signal. When AL and DL are in opposing states,the master can use three of the four data lines to request one bit ofthe data returned from the slave. The three data lines may be used toselect the bit the master is requesting and the fourth line may be usedto transmit the data back to the master. A typical bus transaction mayrequire 16 “writes” by the master to the slave and 8 “reads.” Referringto FIG. 2, the time at: T₁ may indicate the address for the next byte orword saved by the slave; T₂ may show that data is saved to address A₁;T₃ may indicate that the master requests data bit d₂ from the slave; andT₄ may indicate that the slave sends data bit d₂ back.

In this system the data may be used to drive a 4-digit, 7-segmentdisplay light-emitting-diode (LED) display, and 4 discrete LEDindicators (see, e.g., FIG. 17, showing one exemplary display board).The latched data output to the slave may be saved and multiplexed to thedisplay by an independent circuit on the slave controller.

The benefit of latching the data allows the master to off-load theprocessing cycles required to multiplex the display, and can reduce itscost or allow for additional functions to be performed. Additionally,because the data is broken into pieces, and in the normal course ofoperating the unit, the master does not need to update all 32 bits ofthe data, the bus architecture allows the master to update only thoseportions of the data that are required, further reducing the processingrequirements of the master.

To achieve a smaller size, the slave can be implemented with aprogrammable device that allows discrete logic to be configuredinternally and combined with an integrated microprocessor. Such a deviceallows bus architecture to be expandable to up to 64 bits and can drivemore complex displays, such as alphanumeric modes.

A slave device which may be used in this application is a CypressSemiconductor product called a Programmable System On A Chip (PSoC). TheCY8C32 family of parts may be used, for example. The master end may beimplemented with a PSoC device; alternatively, a microcontroller, suchas Microchip part number PIC18F2525, may be used, although almost anymicrocontroller will be able to perform these functions provided it canalso handle the functions of controlling the inverter.

Referring to FIG. 3, a schematic diagram showing exemplary slavehardware for using the bus signals in a preferred embodiment is shown.FIG. 4 is a flow chart showing an exemplary process for the mastercontroller to transmit data on the bus, for the preferred embodiment.FIGS. 5 and 6 are schematic diagrams of exemplary configuration for theslave device.

The above description is not intended to limit the meaning of the wordsused in the following claims that define the invention. For example,while various preferred and less preferred embodiments have beendescribed above, persons of ordinary skill in the art will understandthat a variety of other designs still falling within the scope of thefollowing claims may be envisioned and used. It is contemplated thatfuture modifications in structure, function or result will exist thatare not substantial changes and that all such insubstantial changes inwhat is claimed are intended to be covered by the claims.

We claim:
 1. A system for holding and warming food, comprising: acontainer for holding and warming food, the container consisting of300-series stainless steel; and an induction heating unit with aninduction heating circuit for induction heating the container, theinduction heating circuit including an induction coil having 40 or moreturns of a 120 volt coil, wherein the induction coil operates at acurrent of 5-11 amperes.
 2. The system of claim 1, wherein the inductioncoil comprises 14 gauge wire.
 3. The system of claim 1, furthercomprising a device for measuring the temperature of the container. 4.The system of claim 3, wherein the measured temperature is used in aclosed loop control to limit and maintain the temperature of thecontainer at a desired value.
 5. The system of claim 4 wherein themeasured temperature is used to generate a ramped inverter power profilefacilitating lateral heat transfer in the container.
 6. The system ofclaim 3, wherein the device for measuring the temperature of thecontainer is mounted beneath the container in a manner to facilitatemaintaining contact with the container despite variations in shape orsize of the container.
 7. The system of claim 1, wherein the containeris induction-heated in a manner that minimizes internal heat generationwithin the induction heating unit and thereby enables induction heatingwithout the need for a cooling fan.
 8. A system for holding and warmingfood, comprising: a container for holding and warming food, thecontainer consisting of 300-series stainless steel; and an inductionheating circuit for induction heating the container, the inductionheating circuit including an induction coil having 75 or more turns of a240 volt coil, wherein the induction coil operates at a current of 4-8amperes.
 9. The system of claim 8, further comprising a device formeasuring the temperature of the container.
 10. An apparatus for warmingfood in a 300-series stainless steel container, the apparatuscomprising: an induction heating unit for heating the container byinduction heating, the induction heating unit including an inverterpower circuit, power circuit controls, and an induction coil, theinduction coil having 40 or more turns of a 120 volt coil and aninductance of at least 250 μHenries, wherein the induction coil operatesat a current of 5-11 amperes, and wherein the container consists of300-series stainless steel.
 11. The apparatus of claim 10, wherein theinverter power circuit includes a resonant capacitor.
 12. The system ofclaim 1, wherein the induction coil has an inductance of at least 250μHenries.
 13. The system of claim 3, wherein the device for measuringthe temperature of the container comprises a resistive thermal device.14. The system of claim 9, wherein the device for measuring thetemperature of the container comprises a resistive thermal device. 15.An apparatus for warming food in a 300-series stainless steel container,the apparatus comprising: an induction heating unit with an inductionheating circuit for induction heating the container, the inductionheating circuit including an induction coil having 75 or more turns of a240 volt coil and an inductance of at least 800 μHenries, wherein theinduction coil operates at a current of 4-8 amperes, wherein thecontainer consists of 300 series stainless steel.
 16. The apparatus ofclaim 15, wherein the induction coil comprises 18 gauge wire.
 17. Thesystem of claim 8, wherein the induction coil has an inductance of atleast 800 μHenries.
 18. The apparatus of claim 10, wherein the inductioncoil comprises 14 gauge wire.
 19. A system for holding and warming food,comprising: a container for holding and warming food, the containerconsisting of 300-series stainless steel; and an induction heating unitwith an induction heating circuit for induction heating the container,the induction heating circuit including an induction coil having aninductance of at least 250 μHenries for a 120 volt coil, wherein theinduction coil operates at a current of 5-11 amperes.
 20. The system ofclaim 19, wherein the coil has 40 or more turns.
 21. The system of claim19, wherein the coil comprises 14 gauge wire.
 22. A system for holdingand warming food, comprising: a container for holding and warming food,the container consisting of 300-series stainless steel; and an inductionheating unit with an induction heating circuit for induction heating thecontainer, the induction heating circuit including an induction coilhaving an inductance of at least 800 μHenries for a 240 volt coil,wherein the induction coil operates at a current of 4-8 amperes.
 23. Thesystem of claim 22, wherein the coil has 75 or more turns.
 24. Thesystem of claim 22, wherein the coil comprises 18 gauge wire.